A four wheel steering system steers the front and rear wheels of an automobile. More specifically, an electrically driven, rack and pinion rear wheel steer system produces a desired rear wheel steering angle to improve directional stability at high speeds and steering stability at low speeds. Four wheel steering methods include a xe2x80x9cin phasexe2x80x9d method and a xe2x80x9creverse phasexe2x80x9d steering method. The in phase steering method reduces vehicle yaw (an angular speed about a vertical line passing through a center of gravity of a vehicle) by steering the front and rear wheels in the same direction, and improves vehicle safety due to the reduced yawing. The reverse phase steering method achieves better maneuverability at low speeds by steering the front and rear wheels in differential directions, thereby reducing the turning radius of the vehicle.
In a four wheel or rear wheel steering system a mechanism (such as a spring) may be employed to assist in ensuring that the rear wheels return to a neutral position under inoperative conditions. However, if the rear wheels overshoot an accurate position due to the force of the return spring, undesirable vehicle characteristics and conditions may result.
A vehicle steering system coupled to a steerable wheel, comprising a steering rack linked to said steerable wheel, an electric motor having an output shaft engaged with the steering rack for controlling the steerable wheel. The system also includes a position sensor configured to measure an actual steerable wheel angle of the steering rack and thereby the steerable wheel, a return spring, where the return spring biased so as to provide a returning force to the steering rack. Finally, the system includes a controller that provides a command to the electric motor resulting in a torque, the command is responsive to the actual steerable wheel angle, a desired steerable wheel angle, and a command direction.
A method for controlling an angle of a steerable wheel in a vehicle with electric power steering with an electric motor and a return spring, comprising determining a desired steerable wheel angle, obtaining an actual steerable wheel angle of the vehicle, determining a direction indicative of a command direction for the electric motor, generating the command, responsive to the desired steerable wheel angle, the actual steerable wheel angle, and the command direction. Where the command results in a torque generated by the electric motor operably connected to the steerable wheel for controlling the actual steerable wheel angle.